Technical Field
The present invention relates to a power-module substrate with heat-sink and a manufacturing method thereof for a semiconductor device controlling large electric current and high voltage.
Priority is claimed on Japanese Patent Application No. 2013-212871, filed Oct. 10, 2013, and Japanese Patent Application Nos. 2014-1719 and 2014-1720, filed Jan. 8, 2014, the content of which is incorporated herein by reference.
Background Art
As a power module, a power-module substrate with heat-sink is used, in which an aluminum plate is bonded on a ceramic substrate made of aluminum nitride or the like, and an aluminum-based heat sink is bonded on another surface with an aluminum plate therebetween.
Conventionally, the power-module substrate with heat-sink is manufactured as below. First, piling aluminum plates on a first surface and a second surface of a ceramic substrate, with brazing material appropriate for bonding the ceramic substrate and the aluminum plate between the ceramic substrate and the aluminum plates: and while pressing these with a prescribed pressure, heating to temperature or higher for melting the brazing material and then these are cooled. Thus the aluminum plates are bonded on both the surfaces of the ceramic substrate, and the power-module substrate is manufactured.
Next, piling a heat sink on the aluminum plate at the second surface side of the power-module substrate, with brazing material appropriate for bonding the aluminum plate and the heat sink between the aluminum plate and the heat sink: and while pressing these with a prescribed pressure, heating to temperature or higher for melting the brazing material and then these are cooled. Thus the aluminum plate is bonded to the heat sink, and the power-module substrate with heat-sink can be manufactured.
The aluminum plate bonded on the first surface of the power-module substrate with heat-sink is formed into a circuit layer. On this circuit layer, an electronic component such as a power element or the like is mounted by soldering material.
When manufacturing such a power-module substrate with heat-sink, a warp is generated by thermal contraction by cooling after the bonding process since thermal-expansion coefficients are different between members, such as the ceramic substrate and the aluminum plate.
It is disclosed in Patent Document 1 to control a warp amount at high temperature while soldering chips, terminals, heat-radiation plates and the like and a warp amount when a ceramic circuit-substrate is brought back to room temperature so as not to generate warps.
In Patent Document 2, by bonding a circuit metal-plate and a metal radiation-plate with bending a ceramic substrate so as to manufacture a circuit substrate having a warp in which the circuit metal-plate is concavely bent. Generally, a module in which a heat sink and a circuit substrate are bonded is formed by bonding the heat sink so that the module is flat, and used in a state of being bonded to a fixed part. Accordingly, it is disclosed in Patent Document 2 that a generation and growth of cracks can be reduced when it is mounted to the module or under a practical use by forming the concave warp of the circuit metal-plate of the circuit substrate so that compressive stress remains in the circuit substrate when the circuit substrate is fixed flat.
In Patent Document 3, it is described that a warp generated by solder reflowing of a ceramic circuit-substrate and a heat sink is influenced mainly by a volume ratio and a thickness ratio of a metal radiation-plate and a metal circuit-plate; thus it is possible to obtain an appropriate warp-shape in a heating process by setting these structure to an appropriate range.
As above, the warp of the circuit substrate can be reduced by an adjustment of the thicknesses of the heat sink and the aluminum plate on the heat-sink side forming a metal layer with a neutral axis of the ceramic substrate.